The present invention relates to a two-dimensional CCD type solid-state image pickup device, and more particularly to the constitution of metallic wirings formed in the region above the vertical CCD registers for supplying a transfer pulse to the transfer electrodes.
FIG. 4 is a block diagram of a general interline CCD type solid-state image pickup device. The interline CCD type solid-state image pickup device comprises a plurality of photodiodes 101, a plurality of vertical CCD registers 102 which receive electric charges from the photodiodes and transfer them, a horizontal CCD register 103 which receives electric charges from the vertical CCD registers 102 and transfers them, a charge detection part 104 which detects the electric charges transferred by the horizontal CCD register 103, and an output amplifier 105. An image sensing part consists of an effective image sensing region 106 and an optical black region 107 from which light is shielded in order to determine a black reference signal level.
FIGS. 5(a) and 5(b) are sectional views in the horizontal direction of the picture element in the effective image sensing region and the optical black region of a conventional solid-state image sensing device respectively. As a solid-state image pickup device of this kind one may point out the device, for example, disclosed in Technical Digest, pp. 105-108 of the International Electron Devices Meeting, 1992. First, the constitution of a picture element will be described. There is formed a second conductivity type impurity well layer 112, on one surface of a first conductivity type semiconductor substrate 111. A first conductivity type impurity layer 113 constituting a photodiode is formed inside the layer 112, and a second conductivity type impurity layer 114 for suppressing the generation of a dark current is formed on the surface of the layer 112. Further, a first conductivity type impurity layer 115 constituting a vertical CCD register is formed on the surface of the layer 112, and a second conductivity type impurity layer 116 is formed beneath the layer 115. Between the photodiode and the vertical CCD register there is formed a channel stop 118 except for a transfer gate part 117. An insulating film 119 consisting of a silicon dioxide film or a silicon nitride film is formed on the well layer 112 on the surface of the semiconductor substrate 111, and a transfer electrode 120 consisting of a polysilicon film or the like is formed thereon. A metallic wiring 122 consisting of a tungsten film or an aluminum film is formed further on top of an insulating film 121 consisting of a silicon dioxide film or the like, where the metallic wiring 122 is connected to the transfer electrode 120 through a contact hole 123. The metallic wiring 122 is formed so as to cover the vertical CCD register and to have an opening part in the region above the photodiode, and it functions as a bus line for supplying a vertical transfer pulse to the transfer electrode 120 and as a light shielding film for the vertical CCD register. In the picture element of the optical black region (FIG. 5(b)), a light shielding film 125 consisting of an aluminum film or the like is formed further on top of an insulating film 124 consisting of a silicon dioxide film or the like, providing a structure which prevents the incidence of light on the photodiode. Normally, the structures for the picture element of the effective image sensing region and the optical black region are designed to give identical structure and dimension to the photodiode and the CCD register in order to obtain difference only as to the presence or absence of light incidence, so that their constitutions are given a sole difference in whether or not there exists the light shielding film 125.
The pattern for the metallic wirings 122 is determined in consideration of improvement of the sensitivity of the photodiode and reduction of smears, and is normally designed so as to cover well the side faces of the transfer electrode 120. However, at present where the size of each cell is becoming as small as several micrometers, the design margin is such that the metallic wiring projects 0 to 0.3 um or so from the side faces of the transfer electrode 120 in order for the sensitivity of the photodiode to be maintained. Accordingly, the metallic wiring 122 generates a noticeable level difference in the side face parts of the transfer electrode 120 as shown in FIG. 5(b). Due to the misalignment of the mask or the fluctuation in the etching amount there is also a possibility of having a further aggravation in which the step is formed in the shape of an overhang. As a result, in the picture element in the optical black region, there is a problem in that generation of cracks or disconnection at the step part, or excessive reduction, in the thickness of the light shielding film 125, in the side face parts of the metallic wirings, which allows transmission of light and causes variation in the black reference level.
As a method for resolving this problem there may be mentioned the method disclosed in Japanese Laid-Open Patent Application No. Sho 62-145771. This method proposes to prevent the generation of cracks in the light shielding film by completely covering the surface of the photodiode in the optical black region with the transfer electrode to reduce the level difference. This invention makes it possible to prevent leaking-in of light, but it also introduces new problems. Namely, because of the coverage of the entire surface of the photodiode in the optical black region with the transfer electrode, when a nitride film is interposed between the surface of the photodiode and the transfer electrode, the entire surface of the photodiode is covered with the nitride film that does not readily allow hydrogen to pass through, so that the reduction of the dangling bonds by hydrogen annealing which is generally performed at the final stage of the device formation process becomes infeasible. As a result, the photodiode in the optical black region and the photodiode in the effective image sensing region are given mutually different surface conditions, which prevents an exact black reference signal level from being obtained. Moreover, there is generated a problem that the substrate is stressed by the stress in the nitride film, and is made liable to induce faults. This leads to a result that the dark current of the picture element in the optical black region is larger than that of the picture element in the effective image sensing region, preventing the correct black reference level from being obtained.
As another method for preventing transmission of light through the light shielding film, there is a method disclosed in Japanese Laid-Open Patent Application No. Sho 63-266868. This invention proposes to provide a new subsidiary light shielding film between the original light shielding film and the photodiode for the picture element in the optical black region. According to this invention the leaking-in of light can be prevented, but it introduces a problem that the process number is increased and the production cost is raised.